1. Field of the Invention
This invention relates to a nickel electrode for use in an alkaline battery and to a battery using this nickel electrode.
2. Description of the Prior Art
The nickel electrodes heretofore generally in alkaline batteries are in the form of sintered electrodes. A porous substrate is prepared by sintering nickel powder on a perforated steel sheet. The process by which the porous substrate is filled with active material is called impregnation. The impregnation is accomplished by means of four steps carried out in sequence, namely, soaking the substrate in a nitrate solution, converting into hydroxide in alkaline solution, rinsing with water, and drying. This four step process is repeated until the desired amount of active material is obtained.
This impregnating method, however, is highly complicated because of the many inevitable treatment repetitions. This increases the cost of the product. Moreover, since the porosity of the substrate is limited by practical reasons to the maximum of 80%, and since the loading density of active material is very low, batteries produced by this method have at best a maximum energy density as 400 mAh/cc at the best.
Non-sintered electrodes have been widely developed. For example, as disclosed in Japanese patent application Disclosure No. Sho 56(1981)-59460, an electrode is produced by mixing twenty-odd % by weight of graphite powder as a conductive additive with nickel hydroxide powder coated with cobalt hydroxide, extruding the mixture into a strip, and then laminating two layers of this strip to both sides of a nickel plate, which is a current collector. The produced electrode requires a large amount of graphite, which is the conductive additive, just like a pocket-type electrode. The conductive additive contributes nothing to the volume of the electrode so that it makes the volumetric density of the electrode lowered and produces a large amount of carbonate due to decomposition of the graphite. Therefore, the conductive additive can not be used for a battery such as a sealed nickel-cadmium battery, which contains a small amount of electrolyte.
As one solution of this drawback, for example, pasted nickel electrodes which use a metal fiber substrate of a high porosity (95%) in place of the aforementioned substrate and which permit the required loading of the nickel hydroxide powder to be accomplished in one treatment, thus eliminating the repetitive treatments, have been approaching the level of practical use.
The pasted nickel electrode is produced, as disclosed in Japanese patent application Disclosure No. Sho 61(1986)-138,458, by preparing an active material powder from an aqueous nickel nitrate solution and an aqueous sodium hydroxide solution, adding to the active material a CoO additive to form an electroconductive network in the active material, imparting the consistency of paste to the resultant mixture by addition thereto of a viscous liquid having carboxymethyl cellulose dissolved in water, and loading the pasty mix into a fiber substrate. This nickel electrode is less expensive than the sintered electrode and possesses an energy density as high as 500 mAh/cc.
In consequence of the steady trend in recent years to reducing the weight of portable electronic devices, the market needs are growing for electrodes of the class possessing a high energy level approximating 600 mAh/cc. To meet this need, there is a growing necessity for imparting high density to the nickel hydroxide powder itself because the porosity of the substrate has its own limit.
Nickel hydroxide powder of high density has found popular utility as part of the raw material for parkerizing steel sheets. This nickel hydroxide powder is produced by a method which comprises dissolving nickel nitrate or nickel sulfate in a weakly basic aqueous ammonia solution, thereby giving rise to tetra-ammine nickel (II) complex ion in a stable state, and adding to the resultant solution an aqueous sodium hydroxide solution, thereby inducing deposition of nickel hydroxide so gradually as to preclude growth of voids in the particles being deposited. The use of the produced powder in its unmodified form as part of the active material for batteries, however, entails several drawbacks.
For example, the charge-discharge reaction of the nickel hydroxide electrode proceeds because of free passage of protons (H.sup.+) within the nickel hydroxide crystals. When the nickel hydroxide powder increases in density, the compactness of the crystals thereof is heightened so much as to restrict the freedom of passage of protons within the crystals and the specific surface area of the powder is decreased so much as to increase the current density, with the inevitable result that the high oxide, .gamma.-NiOOH, which causes impairment of discharge and life characteristics as manifested in stepped discharge and swelled electrode, will be produced in a large amount.
The swelling of a nickel electrode due to formation of .gamma.-NiOOH is caused by conversion of high density .beta.-NiOOH to low density .gamma.-NiOOH, as disclosed in J. Appl. Electrochem., 16,403 (1986) and J. Power Source, 12,219 (1984)